This application claims priority of Japanese Patent Application Number 2002-056866, filed Mar. 4, 2002, which is hereby incorporated by reference in its entirety.
The present invention relates to a zoom lens used in digital still cameras, video cameras, and other devices equipped with an imaging element such as a CCD. More specifically, the present invention relates to a compact zoom lens suited for digital still cameras, video cameras, and other devices equipped with an imaging element having a high pixel count.
Japanese Laid-Open Patent Publication Number Hei 5-173073 and Japanese Laid-Open Patent Publication Number 6-201993 disclose examples of conventional zoom lenses that include a first lens group having a negative refractive power; a second lens group having a positive refractive power; and a third lens group having a positive refractive power. The three lens groups are formed from at least seven lenses arranged along an optical axis.
In recent years, there has been significant progress in the technology of solid-state imaging elements such as CCD""s that are used in devices such as video cameras. The increase in the density and number of pixels in these devices has led to a growing need for lenses for optical systems with superior optical characteristics. However, as devices such as digital still cameras and video cameras decrease in size, the zoom lenses mounted on these devices must become more compact, thinner, and lighter. Compactness and thinness, especially in terms of the total length of the lens, is particularly important when the zoom lens is mounted on the main camera unit regardless of whether the camera is in use.
A conventional zoom lens uses at least seven lenses, and therefore, the zoom lens has a significant length along its optical axis. As a result, conventional zoom lenses do not meet the demands for a more compact and thinner digital still camera, video camera, and the like.
The present invention provides a zoom lens having a compact, light, and thin design with superior optical characteristics that efficiently correct various aberrations. The zoom lens is used particularly for imaging elements with high pixel counts that meet the following conditions: a zoom magnification of approximately 2xc3x97; a total lens length at imaging of no more than 34 mm; a total thickness along the optical axis of each lens group of no more than 12 mm; a back focus of no more than 3 mm for placement of a low-pass filter or the like; an exit pupil position of at least 120 mm to prevent eclipsing; a lens brightness (F number) at the wide-angle end of the zoom lens of approximately 2.8; distortion (TV distortion) of no more than |1.5%|; and a thin design when stored. The wide-angle end and the telephoto end are imaging positions of the zoom lens.
The zoom lens according to the present invention includes, from the object side to the image plane side: a first lens group having a negative overall refractive power; a second lens group having a positive overall refractive power; and a third lens group having a positive overall refractive power. The first lens group has a first lens and a second lens bonded together and having a negative refractive power. The second lens group has a third lens with a positive refractive power and a fourth lens and a fifth lens. The fourth and fifth lenses are bonded together and have a negative refractive power. The third lens group has a sixth lens having a positive refractive power.
Since the zoom lens of the present invention has a total of six lenses and four lens components, sensitivity to optical axis offsets (eccentricities) between individual lens groups is reduced. Also, the total length and total retracted length are reduced, and the resulting zoom lens is compact and thin.
The first lens is a bi-convex lens positioned toward the object side of the zoom lens and having a positive refractive power. The second lens is a bi-concave lens positioned toward the image plane side of the zoom lens and having a negative refractive power. The zoom lens satisfies the following:
v2xe2x88x92v1 greater than 20xe2x80x83xe2x80x83(1) 
where v1 is the Abbe number of the first lens and v2 is the Abbe number of the second lens. A zoom lens having this structure can effectively correct chromatic aberration over the entire spectrum, can provide good optical properties, and can have a compact design.
Additionally, the third lens can be a bi-convex lens with both surfaces formed as aspherical surfaces. A zoom lens having this structure can provide a lens brightness (F Number) at the wide-angle end of approximately 2.8 and can effectively correct various types of aberration including spherical aberration.
Additionally, the third lens can satisfy the following:
R5 greater than |R6|xe2x80x83xe2x80x83(2) 
where R5 is the curvature radius on the object side of the third lens and R6 is the curvature radius on the image plane side of the third lens. A zoom lens having this structure can further correct various types of aberration.
Additionally, the fourth lens can be a bi-convex lens positioned on the object side of the zoom lens and having a positive refractive power. The fifth lens can be a bi-concave lens positioned on the image plane side of the zoom lens and having a negative refractive power. A zoom lens having this structure can effectively correct spherical aberration and coma. Also, the second lens group can be formed with two lens components so that the space taken up by the lenses is shorter.
Additionally, the sixth lens is a bi-convex lens with both sides formed as aspherical surfaces. A zoom lens having this structure can prevent increased aberration by reducing the total lens length. More specifically, a zoom lens having this structure can effectively correct coma at the telephoto end.
Additionally, the zoom lens can satisfy the following:
0.4 less than f2/|f1| less than 0.8xe2x80x83xe2x80x83(3) 
1.5 less than f3/fw less than 2.1xe2x80x83xe2x80x83(4) 
where fi is the focal length of i-th lens group (i=1-3) and fw is the focal length of the total lens system at the wide-angle end. A zoom lens having this structure can be compact, can provide a zoom factor of approximately 2xc3x97 and good optical properties, and can have an exit pupil position that can be set appropriately.
Additionally, the first lens group, the second lens group, and the third lens group can each move along an optical axis during a zoom operation from the wide-angle end to the telephoto end. A zoom lens having this structure can effectively correct various types of aberration, especially astigmatism, at intermediate focal lengths between the wide-angle end and the telephoto end.
Additionally, the third lens group can move first toward the object side of the zoom lens along the optical axis and then toward the image plane side of the zoom lens during a zoom operation from the wide-angle end to the telephoto end. A zoom lens having this structure can further correct astigmatism at intermediate focal lengths between the wide-angle end and the telephoto end.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.